A) Make sure the Agent folder is backed up, per the instructions found in the upgrade plan. Then uninstall all instances of Spotlight on the server via the Programs and Features tool in Windows.
B) Reinstall the original version of Spotlight prior to upgrade again, but point to a new installation path. For example, if installed in C:\ try D:\ or even change from C:\Quest Software to C:\Quest.
C) Copy the backup agent folder to the new install path agent folder. Restart the Diagnostic Server service and check if all is back to normal.
D) Install the latest version of Spotlight to re-attempt the upgrade.